Received: 8 November 2018
Accepted: 10 July 2019
First Online: 16 July 2019
Ethics approval and consent to participate
: This research does not involve human subjects, human material, or human data. Local ethics committee (Ethics Committee of the School of Public Health, Jilin University) ruled that no formal ethics approval was required in this particular case.
: Not applicable.
: The authors declare that they have no competing interests.